Sensors operating by using the sensing ability of a plate capacitor are known. The medium, a characteristic of which is to be sensed or measured, forms a dielectric between the plates of the plate capacitor. Such capacitive sensors are particularly suitable for sensing the filling level of a liquid such as oil in a container. Basically medium characteristics or parameters other than the filling level can also be derived from the dielectric characteristics of the medium. Tubular capacitors with cylindrical "plates" are also known for such sensing purposes. For example, German Patent Publication DE 196 44 777 C1 describes, in addition to the general prior art, a filling level sensor arrangement with a plurality of capacitive sensors which are electrically interconnected in several groups. The capacitive sensors forming the groups are arranged in stacks on one side of a carrier. The evaluation of the measured signals takes place in a quasi-digital mode for each group of capacitive sensors, whereby the effort and expense for the electrical wiring and for the evaluation circuit arrangement are increased.
European Patent Publication EP 0,401,111 B1, published in French, as translated in German Patent Publication DE 690 01 151 T2 discloses a capacitive sensor arrangement of the type to which the present invention relates. Two carrier plates carry metal capacitor coatings or claddings on plate surfaces facing each other. The carrier plates are interconnected by several rivets. Spacer rings are positioned on the rivet shafts between the plates. The rivets and the spacer rings are necessarily positioned outside the areas of the metal capacitor coatings or claddings. Further, the rivets and spacer rings must be electrically insulated from the metal capacitor coatings or cladding. These features of the known sensor increase its costs.
The carrier plates referred to hereafter as circuit boards or simply as boards are made of printed circuit board material and etching methods are used to structure the metal capacitor coatings or claddings and the required conductor leads. The boards carry as part thereof several coatings forming capacitor plates spaced from one another by the rings on the rivets and include for example a main electrode for sensing a filling level and a dielectric measuring electrode for measuring a possibly changing dielectric between the capacitor plates. A further metal coating may be provided to form an electric screen. These various coatings or cladding elements are individually connected by leads to respective connector areas on the boards. These connector areas and possibly further elements on the boards are connected to a separately arranged electronic measuring device or signal evaluating circuit not shown in any detail. The electric conductors required for connecting the sensor to a remote signal evaluating circuit are trouble prone and costly.
Besides, the measuring accuracy of the known sensor leaves room for improvement. In the known sensor the measuring accuracy depends much on the structural stability of the sensor namely on the number of rivets and the spacing of the rivets from each other. The rivets cannot assure in all instances that the boards will not bend or warp. Such warping is undesirable because it varies the spacing between the capacitor plates or rather metal coatings on the boards. A non-constant spacing varies the measuring accuracy.